1. Field of the Invention
The present invention relates to the detection of constituents in a test sample. More particularly, the invention relates to the qualitative and semi-quantitative analysis of a sample for constituents which possess peroxidative activity.
2. Description of the Prior Art
Many analytical methods are presently available for detecting the presence of peroxidatively active substances in samples such as urine, fecal suspensions, and gastrointestinal contents. Hemoglobin and its derivatives are typical of such "peroxidatively active" substances because they behave in a manner similar to the behavior of the enzyme peroxidase. Such substances have also been referred to as pseudoperoxidases. Peroxidatively active substances are enzyme-like in that they catalyze the redox reaction between peroxides and benzidine, o-tolidine, 3,3',5,5'-tetramethylbenzidine, 2,7-diaminofluorene or similar indicator substances, thereby producing a detectable response such as a color change. Hence, most methods for determining the presence of occult blood in test samples rely on this pseudoperoxidase activity.
Several methods have evolved over the years which rely on enzyme-like catalysis of the peroxidic oxidation of color-forming indicators. Primarily these include wet chemical procedures and "dip-and-read" type reagent-bearing strips. Of the former, a typical example is set forth in Richard M. Henry, et al., Clinical Chemistry Principles and Techniques (Hagerstown, Maryland: Harper and Row, 1974), pp. 1124-1125. This procedure involves the use of glacial acetic acid (buffer), diphenylamine (indicator), and hydrogen peroxide. While such wet methods have proven analytical ability, they are nevertheless fraught with obvious shortcomings, not the least of which are poor reagent stability and inadequate sensitivity. Inherent to such reagent solutions is a decline in stability (ergo sensitivity) so rapid that fresh reagent solutions must be prepared after several days of storage, a necessity resulting in both excessive time required of analytical personnel, and poor economy because of having to waste costly reagents.
A second method for the determination of peroxidatively active substances, and the one presently preferred by most clinical assayists and analysts, is the use of so-called "dip-and-read" reagent strips. Typical of such devices is a reagent strip manufactured by the Ames Company Division of Miles Laboratories, Inc. and sold under the name HEMASTIX.RTM.. This reagent strip comprises, in essence, a porous paper matrix affixed to a plastic strip or handle. The matrix is incorporated with a buffered mixture of an organic hydroperoxide and o-tolidine. Upon immersion in a liquid containing hemoglobin, myoglobin, erythrocytes or other pseudoperoxidases, a blue color develops in the matrix, the intensity of which is proportional to the concentration of the peroxidatively active substance in the sample. Thus, by comparing the color developed in the matrix to a standard color chart, the assayist can determine, on a semi-quantitative basis, the amount of unknown present in the sample.
Hence, the advantages of reagent strips over wet chemistry methods are predominantly twofold: strips are easier to use because neither the preparation of reagents nor the attendant apparatus are required; and greater stability of reagents is afforded, resulting in greater accuracy, sensitivity and economy.
But the inherent advantages of strips over wet chemistry notwithstanding, certain characteristics of presently available strips are in need of improvement. These are stability and sensitivity. Whereas these properties in current state-of-the art strips for determining pseudoperoxidases are greatly enhanced over those of wet chemical methods, there would nevertheless accrue a great advance in the art if such strips could be made even more stable during storage and even more sensitive to peroxidatively active substances. It was towards achieving these improvements that the research activities resulting in the present invention were directed.
At least three attempts at achieving the above-mentioned goals are recorded in the prior art. A recitation in Chemical Abstracts Volume 85, page 186 (1976) describes a two-dip method for preparing reagent strips containing o-tolidine and phenylisopropyl hydroperoxide. In this method a solution is made of the indicator (o-tolidine .multidot. 2HCl) and polyvinylpyrrolidone in ethanol). To this solution were added a small amount of surfactant and enough citrate buffer to provide a pH of 3.7. Filter paper strips impregnated with ethyl cellulose were dipped in this solution and dried. The thus-impregnated filter paper was subsequently dipped into a second solution containing 1,4-diazabicyclo [2.2.2]octane, phenylisopropyl hydroperoxide and polyvinylpyrrolidone dissolved in an ethanol-toluene mixture. The thrust of this experiment was to stabilize the peroxide and indicator combination through the use of the bicyclooctane derivative and the polyvinylpyrrolidone.
A second such method is disclosed in U.S. Pat. No. 3,853,471. This patent teaches the use of phosphoric or phosphonic acid amides where the substituent amido groups are primarily N-morpholine radicals.
Besides these attempts, there also exists the disclosure of U.S. Pat. No. 3,252,762 wherein the organic hydroperoxide is physically encapsulated within a colloidal material such as gelatin. Thus, when such a test strip is utilized, the aqueous test sample dissolves the gelatin capsules, thereby freeing the hydroperoxide for further reaction with the indicator in the presence of a peroxidatively active substance.
Each of these prior attempts was aimed at stabilizing the reagents so that the potentially incompatible reactive ingredients (hydroperoxide and indicator) would not prematurely combine and thereby render the test strips less sensitive. Hence, it can be said that the prior art methods were not directed towards the combined objectives of simultaneously enhancing stability and sensitivity, but rather they attempted to preserve existing sensitivity by preventing reagent decomposition during storage.
Another prior art reference which is of interest to the general concepts discussed herein in U.S. Pat. No. 3,236,850. This patent is directed towards stabilizing organic hydroperoxides used as catalysts and oxidizing agents. The patentees in this reference disclose the use of primary, secondary, or tertiary amine salts with organic peroxides. This reference is in no way directed toward reagent test strips.
Upon realizing that none of the above-described methods would achieve the kind of stability and sensitivity desired in a test strip for detecting peroxidatively active substances, the present inventor decided to take a completely different tack. This different approach was discovered during the research which led to the present invention, and resulted in a composition and device which completely fulfilled the desired objectives of increased stability and sensitivity.
But, even more surprisingly, yet another advantage resulted from this work -- an improved method for preparing the device presently disclosed whereby the manufacture is dramatically simpler than processes enumerated in the foregoing prior art references -- a one-dip method.